1. Field of the Invention
This invention relates to directional couplers in general and more particularly to a directional coupler for low frequencies that has good power handling and a small package size.
2. Description of Related Art
Directional couplers are used in a variety of applications in the RF and microwave frequency range. FIG. 1 shows a schematic diagram of a prior art directional coupler 20 including a pair of coupled circuit lines 22 and 24. Circuit lines 22 and 24 would typically be formed in a stripline configuration. The directional coupler 20 has four ports, an input port 25, an output port 26, a forward coupled port 27 and a reverse coupled port 28. An input signal or power applied to the input port 25 will go mainly to the output port 26. A portion of the input signal will be electromagnetically coupled to circuit line 24 and appear mostly at forward coupled port 27. A very small portion of the signal will go to the coupled reverse port 28.
The electrical signal coupled to the forward and reverse ports depends upon the coupled circuit line characteristic impedance and the coupling between the lines. Directivity is a measure of the bi-directional coupler differentiation.
Directional couplers using stripline configurations have been applied to higher frequency applications, typically above 600 MHz. The length of the coupled lines is typically set at one quarter of the wavelength at the center frequency. The directional coupler 20 of FIG. 1 is impractical for higher frequency applications. Directional couplers operating at lower frequencies are often faced with size and space constraints, which require the use of transformers to handle the power levels. The use of transformers add higher costs to the product and result in a larger overall package.
A current unmet need exists for a directional coupler that can operate at low frequencies, with minimal size and improved electrical performance.